"Nurse Jackie" administers an odd cocktail -- one that starts out potent but loses some fizz with each successive round.

“Nurse Jackie” administers an odd cocktail — one that starts out potent but loses some fizz with each successive round. “The Sopranos'” Edie Falco is clearly the straw that stirs the drink as a cranky, pill-popping nurse surrounded by an assortment of eccentric characters. Yet after the promising, twisty premiere (which makes especially good use of the theme from “Valley of the Dolls”), the series becomes hard-to-define in mostly the wrong ways — a half-hour that’s not particularly funny, simply dark and bleak, yet without much high-stakes drama. Alas, even Showtime can’t quite live by “quirky” alone.

“Quiet and mean. Those are my people,” Falco’s nurse, Jackie Peyton, announces to a fresh-faced nursing student (Zoey Barkow) in the premiere, directed by another “Sopranos” alum, Allen Coulter.

Although she’s told early on by a friendly doctor (Eve Best) that she’s “the only sane one there,” Jackie’s sanity comes with strings attached. For starters, she’s traded in booze for an addiction to painkillers, to which she has ready access thanks to her relationship with the hospital pharmacist (Paul Schulze).

Unlike the tendency of medical shows to be doctor-centric, “Nurse Jackie” advances the premise that it’s the nurses who really know what’s up, and essentially run the place. Indeed, Jackie announces that a handsome young M.D. (Peter Facinelli) is clueless “when it comes to actual patients,” and commiserates with another nurse (“The Visitor’s” Haaz Sleiman) about doctors’ general stupidity.

Part of the show’s problems might stem from expectations. Despite being a half-hour, “Jackie” is virtually laugh-free, playing like a half-hour drama. Indeed, at times the dour tone makes its lead-in, “Weeds,” seem positively cheerful by comparison.

While the title character is consistently rough and the language blue, in subsequent episodes (Showtime sent six out for review) the series increasingly feels like all style and limited substance — a star showcase that’s less “triumphant return” than “Nice to have you back, but … .”

That’s disappointing, since Showtime achieved a genuine breakthrough with “United States of Tara,” creating a fabulous vehicle for Toni Collette. The trick, though, was that show eclipsed its gimmicky premise with a depth and heart that this one initially lacks. Then again, perhaps that’s because the program approaches its storylines with the same sense of numb detachment Jackie is feeling (or rather, not feeling).

Mostly, “Nurse Jackie” plays like TV designed for theater folk, capitalizing on its New York base by casting performers such as Best (a two-time Tony nominee) and Judith Ivey, Swoosie Kurtz and Blythe Danner in guest roles in a later episode. While all that talent is welcome, it’s a narrow foundation even by pay-cable standards.

Exec producers John Melfi, Liz Brixius and Linda Wallem previously worked on HBO’s “The Comeback,” which also trafficked primarily in discomfort before coming to a surprisingly satisfying conclusion — and who knows, they might do so again. After all, “Nurse Jackie” would appear to possess the requisite ingredients for success. What it’s lacking, thus far, is a prescription for putting them all together.